In 23 Munich-Wistar rats with surface glomeruli, the determinants of glomerular ultrafiltration and peritubular capillary uptake of proximal reabsorbate were studied before and during intra-arterial infusions of mildly vasodepressor doses of prostaglandin E1,acetylcholine, and bradykinin. For each drug single-nephron glomerular filtration rate remained unchanged from normal hydropenic values while glomerular plasma flow rate increased, resulting in declines in single-nephron filtration fraction (SNFF). Mean glomerular transcapillary hydraulic pressure difference (delta P) increased or remained unchanged on average. Declines in SNFF were accompanied by reductions in efferent arteriolar oncotic pressure (piE). Filtration pressure equilibrium, equality between pi E and delta P, obtained before but not during drug infusions. In the latter situation values for the glomerular capillary ultrafiltration coefficient were calculated and found to be significantly reduced from published control values. Despite marked falls in pi E during drug infusion, absolute proximal reabsorption was not reduced significantly, due, it is suggested, to the opposing effects of increases in efferent arteriolar plasma flow and interstitial hydraulic pressure.